Optical fiber having low non-linearity for WDM transmission

ABSTRACT

An optical transmission fiber has a refractive index profile with an area of increased index of refraction at the inner core of the fiber, an annular region positioned radially outward from the inner core with an index of refraction exceeding the index of the inner core, and at least a low dopant content region in a cross-sectional region between the inner core and the annular region. A low loss cladding layer surrounds the core region. The optical transmission fiber with this segmented core profile provides a high effective area, low non-linearity coefficient, nonzero dispersion, and relatively flat dispersion slope.

Under provisions of 35 U.S.C. §119(e), the applicants claim the benefitof U.S. provisional application No. 60/090,791, filed Jun. 25, 1998,which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to an optical transmission fiberthat has improved characteristics for minimizing non-linear effects, andspecifically to an optical fiber for use in awavelength-division-multiplexing (WDM) system that has two refractiveindex peaks with the maximum index of refraction difference located inan outer core region.

In optical communication systems, non-linear optical effects are knownto degrade the quality of transmission along standard transmissionoptical fiber in certain circumstances. These non-linear effects, whichinclude four-wave mixing (FWM), self-phase modulation (SPM), cross-phasemodulation (XPM), modulation instability (MI), stimulated Brillouinscattering (SBS) and stimulated Raman scattering (SRS), particularlycause distortion in high power systems. The strength of non-lineareffects acting on pulse propagation in optical fibers is linked to theproduct of the non-linearity coefficient γ and the power P. Thedefinition of the non-linearity coefficient, as given in the paper“Nonlinear pulse propagation in a monomode dielectric guide” by Y.Kodama et al., IEEE Journal of Quantum Electronics, vol. QE-23, No. 5,1987, is the following: $\begin{matrix}{\gamma = {\frac{1}{\lambda \quad n_{eff}}\quad \frac{\int_{0}^{\infty}{{n(r)}{n_{2}(r)}{{F(r)}}^{4}r{r}}}{\lbrack {\int_{0}^{\infty}{{{F(r)}}^{2}r{r}}} \rbrack^{2}}}} & (1)\end{matrix}$

where r is the radial coordinate of the fiber, n_(eff) is the effectivemode refractive index, λ is a signal wavelength, n(r) is the refractiveindex radial distribution, n₂(r) is the non-linear index coefficientradial distribution, and F(r) is the fundamental mode radialdistribution.

Applicants have identified that equation (1) takes into account theradial dependence of the non-linear index coefficient n₂ which is due tothe varying concentration of the fiber dopants used to raise (or tolower) the refractive index with respect to that of pure silica.

If we neglect the radial dependence of the non-linear index coefficientn₂ we obtain a commonly used expression for the coefficient γ.$\begin{matrix}{\gamma = \frac{2\pi \quad n_{2}}{\lambda \quad A_{eff}}} & (2)\end{matrix}$

where we have introduced the so called effective core area, or briefly,effective area, $\begin{matrix}{A_{eff} = {\frac{2\quad {\pi \lbrack {\int_{0}^{\infty}{{{F(r)}}^{2}r{r}}} \rbrack}^{2}}{\int_{0}^{\infty}{{{F(r)}}^{4}r{r}}}.}} & (3)\end{matrix}$

The approximation (2), in contrast to the definition (1) does notdistinguish between refractive index radial profiles that have the sameeffective core area A_(eff) value but different γ values. While1/A_(eff) is often used as a measure of the strength of non-lineareffects in a transmission fiber, γ as defined by equation (1) actuallyprovides a better measure of the strength of those effects.

Group velocity dispersion also provides a limitation to qualitytransmission of optical signals across long distances. Group velocitydispersion broadens an optical pulse during its transmission across longdistances, which may lead to dispersion of the optical energy outside atime slot assigned for the pulse. Although dispersion of an opticalpulse can be somewhat avoided by decreasing the spacing betweenregenerators in a transmission system, this approach is costly and doesnot allow one to exploit the advantages of repeaterless opticalamplification.

One known way of counteracting dispersion is by adding suitabledispersion compensating devices, such as gratings or dispersioncompensating fibers, to the telecommunication system.

Furthermore, to compensate dispersion, one trend in opticalcommunications is toward the use of soliton pulses, a particular type ofRZ (Return-to-Zero) modulation signal, that maintain their pulse widthover longer distances by balancing the effects of group velocitydispersion with the non-linear phenomenon of self-phase modulation. Thebasic relation that governs soliton propagation in a single mode opticalfiber is the following: $\begin{matrix}{{P_{0}T_{0}^{2}} = {{cost}\quad \frac{D\quad \lambda^{2}}{\gamma}}} & (4)\end{matrix}$

where P₀ is the peak power of a soliton pulse, T₀ is the time durationof the pulse, D is the total dispersion, λ is the center wavelength ofthe soliton signal, and γ is the previously introduced fibernon-linearity coefficient. Satisfaction of equation (4) is necessary inorder for a pulse to be maintained in a soliton condition duringpropagation.

A possible problem that arises in the transmission of solitons inaccordance with equation (4) is that a conventional optical transmissionfiber is lossy, which causes the peak power P₀ of the soliton pulse todecrease exponentially along the length of the fiber between opticalamplifiers. To compensate for this decrease, one can set the solitonpower P₀ at its launch point at a value sufficient to compensate for thesubsequent decrease in power along the transmission line. An alternativeapproach, as disclosed for example in F. M. Knox et al., paper WeC.3.2,page 3.101-104, ECOC, '96, Oslo (Norway), is to compensate (withdispersion compensating fiber, although fibre Bragg gratings can also beused) for the dispersion accumulated by the pulses along the stretchesof the transmission line where the pulses' peak power is below a solitonpropagation condition.

Optical fibers having a low non-linearity coefficient are preferred foruse in transmission systems, such as Non-Return-to-Zero (NRZ) opticallyamplified WDM systems, as well as non amplified systems, to avoid orlimit the non-linear effects mentioned above. Furthermore, fibers with alower non-linearity coefficient allow an increase in the launch powerwhile maintaining non-linear effects at the same level. An increasedlaunch power in turn means a better S/N ratio at the receiver (lowerBER) and/or the possibility to reach longer transmission distances byincreasing the amplifier spacing. Accordingly, Applicants have addresseda need for optical fibers having low values of non-linearity coefficientγ.

Also in the case of soliton systems, to increase the spacing betweenamplifiers one can increase the launch power for the pulses using morepowerful amplifiers. In this case, however, equation (4) implies that ifthe launch power is increased and the soliton pulse duration remainsconstant, the ratio D/λ²/γ must accordingly be increased. Therefore,lower values of non-linear coefficient γ are desirable also to providean increased distance between line amplifiers in a soliton transmissionsystem.

Patents and publications have discussed the design of opticaltransmission fibers using a segmented core or double-cladding refractiveindex profile and fibers having a large effective area. For example,U.S. Pat. No. 5,579,428 discloses a single-mode optical fiber designedfor use in a WDM soliton telecommunication system using optical lumpedor distributed amplifiers. Over a preselected wavelength range, thetotal dispersion for the disclosed optical fiber lies within apreselected range of positive values high enough to balance self-phasemodulation for WDM soliton propagation. As well, the dispersion slopelies within a preselected range of values low enough to preventcollisions between WDM solitons and to reduce their temporal andspectral shifts. The proposed fiber of the '428 patent is a segmentedcore with a region of maximum index of refraction in the core of thefiber.

U.S. Pat. No. 4,715,679 discloses an optical fiber having a segmentedcore of a depressed refractive index for making low dispersion, low losswaveguides. The '679 patent discloses a plurality of refractive indexprofiles including an idealized profile having an area of maximum indexof refraction at an annular region outside the inner core of the fiberbut inside an outer core annular region.

U.S. Pat. No. 4,877,304 discloses an optical fiber that has a coreprofile with a maximum refractive index greater than that of itscladding. U.S. Pat. No. 4,889,404 discloses an asymmetricalbidirectional optical communication system including an optical fiber.While the '304 and '404 patents also describe idealized refractive indexprofiles potentially having an outer annular region with an increasedindex of refraction, no specific examples corresponding to thoseprofiles are disclosed and the patents are silent as to the non-linearcharacteristics of optical fibers having those profiles.

U.S. Pat. No. 5,684,909, EP 789,255, and EP 724,171 disclose single modeoptical fibers having large effective areas made by a segmentedrefractive index core profile. This patent and applications describecomputer simulations for obtaining fibers with a large effective areafor use in long distance, high bit rate optical systems. The '909 patentshows a core profile having two non-adjacent profile segments having apositive index of refraction and two additional non-adjacent segmentshaving a negative index of refraction. The '909 patent aims to achieve afiber with a substantially zero dispersion slope from the segmented coreprofile. The fibers disclosed in EP 789,255 have extremely largeeffective areas achieved by a refractive index profile with a segmentedcore but having at least two non-adjacent segments with negativerefractive difference. EP 724,171 discloses optical fibers with themaximum index of refraction present at the center of the fiber.

U.S. Pat. No. 5,555,340 discloses a dispersion compensating opticalfiber having a segmented core for obtaining dispersion compensation. The'340 patent discloses a refractive index profile where a resin filmsurrounding a cladding has a higher index of refraction than the innercore of the fiber. This resin, however, does not serve as a low-losslight-conductive layer in the fiber structure.

SUMMARY OF THE INVENTION

Applicants have noticed that the distribution ofrefractive-index-modifying dopants in the fiber cross-section has asignificant impact on the fiber non-linearity characteristics.Applicants have determined that the non-linear index n₂ contributes tothe non-linearity coefficient γ with a constant term, due to pure silicaand with a radially varying term, proportional to the concentration ofindex-modifying dopants. Dopants that are added to pure silica glass toincrease the refractive index (e.g., GeO₂) or to decrease it (e.g.,fluorine) both tend to increase the glass non-linearity beyond thenon-linearity value of pure silica. Applicants have found that knownlarge-effective-area fibers, while achieving an overall increase ineffective area, fail to achieve an optimum decrease in γ, due to theeffect of dopants in areas of the fiber cross-section where the opticalfield has a relatively high intensity.

Furthermore, Applicants have noticed thatrefractive-index-modifying-dopants tend to increase fiber loss, inparticular due to an increased scattering loss. According to the above,Applicants have afforded the task of developing an optical fiber with alow non-linearity coefficient γ and a limited loss.

Applicants have developed an optical fiber having a comparatively lowdopant concentration where the optical field intensity is relativelyhigh, and a comparatively higher dopant concentration where the opticalfield intensity is relatively low.

Applicants have found that a low non-linearity coefficient γ can beachieved in an optical fiber by selecting an index profile for the fiberwith a first peak in the fiber central cross-sectional area, an outsidering with a second peak value higher than the first peak and at least alow-dopant-content region in a cross-sectional region between the twopeaks. In this fiber the optical field intensity outside the inner coreregion is increased. The presence of a low-dopant-content region incombination with a relatively high field intensity achieves asubstantial decrease in the non-linearity coefficient, together with alimited impact on fiber loss.

In one aspect, an optical transmission fiber with a low non-linearitycoefficient γ and high effective area consistent with the presentinvention includes a core region and a low loss cladding surrounding thecore region. The core region comprises: a glass inner core having afirst maximum refractive index difference Δn1, a profile α, and a radiusr1; a first glass layer radially surrounding the inner core, having asubstantially constant refractive index difference Δn2 less than Δn1,and having an outer radius r2; and a second glass layer radiallysurrounding the first layer, having a second maximum refractive indexdifference Δn3 greater than Δn1, and having a width w at the base,wherein γ is less than about 2 W⁻¹ km⁻¹ over a preselected operatingwavelength range. The refractive index difference Δn2 of the first glasslayer is lower in absolute value than 10% of said second maximumrefractive index difference Δn3. More preferably Δn2 is lower inabsolute value than 5% of Δn3. Preferably Δn2 is substantially constantacross the first glass layer.

Preferably the peak index of refraction Δn3 of second glass layerexceeds the peak index of refraction Δn1 for inner core by more than 5%.

In a second aspect, an optical transmission fiber with a high effectivearea and a non-linearity coefficient γ lower than about 2 W⁻¹km⁻¹ foruse in an optical transmission system consistent with the presentinvention has a core region and a low loss cladding surrounding the coreregion. The core region includes a glass inner core having a firstmaximum refractive index difference Δn1, a profile α, and a radius r1; afirst glass layer radially surrounding the inner core, having arefractive index difference Δn2 less than Δn1, and having an outerradius r2; and a second glass layer radially surrounding the firstlayer, having a second maximum refractive index difference Δn3 greaterthan Δn1, and having a width w. Said first glass layer comprises alow-dopant-content region.

In a further aspect, an optical transmission system consistent with thepresent invention comprises an optical transmitter for outputting anoptical signal and an optical transmission line for transmitting saidsignal. The optical transmission line comprises an optical transmissionfiber having a first refractive index peak in the fiber centralcross-sectional area, an outside ring with a second refractive indexpeak value higher than the first peak and a low-dopant-content regionbetween the two peaks.

Preferably said low-dopant-content region has a refractive indexdifference, in absolute value, of about, or lower than, 15% of the fiberpeak refractive index difference, i.e. of the refractive indexdifference of the outside ring.

In a preferred embodiment, the optical transmission system furthercomprises a plurality of optical transmitters for outputting a pluralityof optical signals, each signal having a particular wavelength, and anoptical combiner for combining the optical signals to form a wavelengthdivision multiplexed optical communication signal and outputting thecombined signal onto said optical transmission line.

Preferably, said optical transmission fiber has a length greater than 50km.

Preferably, said optical transmission line comprises at least oneoptical amplifier.

In a still further aspect, a method consistent with the presentinvention for controlling non-linear effects in optical fibertransmission comprises the steps of: generating an optical signal;coupling the optical signal in a silica optical fiber having anon-linearity coefficient; doping a central cross-sectional area of thefiber to provide a first refractive index peak; enhancing a fieldintensity associated with the optical signal in a fiber cross-sectionalarea outside said central cross-sectional area, by doping an annularglass ring of said fiber to provide a second refractive index peakvalue, higher than the first peak. The method comprises the step ofselecting a dopant concentration of a fiber cross-sectional regionbetween the two peaks below a predetermined value, so as to reduce thefiber non-linearity coefficient.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed. The followingdescription, as well as the practice of the invention, set forth andsuggest additional advantages and purposes of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description, explain the advantages and principles ofthe invention.

FIG. 1 is a cross-section of an optical transmission fiber consistentwith the present invention;

FIG. 2 is a graph of the refractive index profile of the cross-sectionof the fiber in FIG. 1 consistent with a first embodiment of the presentinvention;

FIG. 3 is a graph of computer simulations of dispersion vs. inner-coreradius for the first embodiment of the present invention;

FIG. 4 is a graph of computer simulation of effective area vs. area ofrefractive index profile for inner core for the first embodiment of thepresent invention;

FIG. 5 is a graph of computer simulation of non-linearity coefficient γvs. inner peak area for the first embodiment of the present invention;

FIG. 6 is a graph of computer simulations of effective area vs. index ofrefraction for the second glass layer for the first embodiment of thepresent invention;

FIG. 7 is a graph of computer simulation of electric field vs. opticalfiber radius for the first embodiment of the present invention;

FIG. 8A is a graph of computer simulations of non-linearity coefficientvs. effective area for the first embodiment of the present invention;

FIG. 8B is a graph of computer simulations of non-linearity coefficientvs. effective area for conventional dual-shape dispersion-shiftedoptical fibers;

FIG. 9 is a graph of the refractive index profile of the cross-sectionof the fiber in FIG. 1 consistent with a second embodiment of thepresent invention;

FIG. 10 is a graph of the refractive index profile of the cross-sectionof the fiber in FIG. 1 consistent with a third embodiment of the presentinvention;

FIG. 11 is a refractive index profile of the optical fiber in FIG. 1consistent with a fourth embodiment of the present invention;

FIG. 12 is a graph of total dispersion vs. wavelength for a fiberaccording to the fourth embodiment of the present invention;

FIG. 13 is a refractive index profile of the fiber in FIG. 1 consistentwith a fifth embodiment of the present invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to various embodiments according to thisinvention, examples of which are shown in the accompanying drawings andwill be obvious from the description of the invention. In the drawings,the same reference numbers represent the same or similar elements in thedifferent drawings whenever possible.

Optical fibers consistent with the present invention have a refractiveindex profile that includes two areas of peak refractive indexdifference in a radial dimension of the fiber, where the greater of thetwo peaks is positioned radially outward from the first peak. Applicantshave discovered that optical fibers having refractive index profiles ofthis nature can produce optical characteristics in a wavelengthoperating range of 1520 to 1620 nm that includes a relatively lownon-linearity coefficient γ and a relatively high effective area. Due totheir characteristics, the invention fibers can be advantageously used,in particular, in long length (e.g., greater than 50 km) opticaltransmission lines and/or with high power signals (e.g., in opticaltransmission lines with optical amplifiers). Moreover, Applicants havediscovered that optical fibers including this refractive index profilecan effectively operate as nonzero dispersion fibers to minimize thenon-linear effect of four-wave mixing in WDM systems, both for nonzeropositive dispersion and nonzero negative dispersion. Furthermore,Applicants have determined that optical fibers including this refractiveindex profile can effectively operate as dispersion shifted fibers tominimize the non-linear effects in optical transmission systems.

As generally referenced as 10 in FIG. 1, the optical transmission fiberwith a low non-linearity coefficient γ comprises a plurality of lightconducting layers of glass with varying indices of refraction. As shownin the cross-section of fiber 10 in FIG. 1, the axial center of thefiber is an inner core 12 that has a first maximum refractive indexdifference Δn1 and a radius r1. As readily known to those of ordinaryskill in the art, refractive index difference refers to the differencein refractive index between a given layer of glass and the claddingglass. That is, the refractive index difference Δn1 of inner core 12,having a refractive index n1, equals n1−ncladding. Glass core 12preferably is made of SiO₂ doped with a substance that increases therefractive index of pure SiO₂, such as GeO₂. Other dopants increasingthe refractive index are for example Al₂O₃, P₂O₅, TiO₂, ZrO₂ and Nb₂O₃.

A first glass layer 14 surrounds the inner core 12 and is characterizedby an index of refraction across its width that is less than the indicesof refraction along the radius r1 of inner core 12. Preferably, and asdiscussed in more detail below, first layer 14 is made of pure SiO₂ thathas a refractive index difference Δn2 substantially equal to 0.

A second glass layer 16 surrounds the first glass layer 14 along thelength of the fiber 10. Second glass layer 16 has a maximum index ofrefraction Δn3 within its width that exceeds the maximum index ofrefraction of the glass Δn1 within inner core 12. Finally, a low losscladding 18 surrounds the second glass layer 16 in a conventional mannerto help guide light propagating along the axis of fiber 10. Cladding 18may comprise pure SiO₂ glass with a refractive index differencesubstantially equal to 0. If cladding 18 includes somerefractive-index-modifying dopant, the cladding should have an index ofrefraction across its width that is less than the maximum indices ofrefraction within both inner core 12 and second layer 16.

FIG. 2 illustrates a refractive index profile across the radius of fiber10 for a first embodiment of the present invention. As generally shown,10 has two refractive index peaks 20 and 22 positioned respectivelywithin inner core 12 and second layer 16. First layer 14, which isdisposed radially between inner core 12 and second layer 16, provides arefractive index dip relative to its two adjacent layers 12 and 16.Consequently, the combination of inner core 12, first layer 14, andsecond layer 16 generally provides an optical fiber profile having asegmented core with an outer layer having the highest index ofrefraction within the cross-section of the fiber.

As shown in FIG. 2, according to a first embodiment of the presentinvention, inner core 12 has a radius r1 that is about 3.6 μm to 4.2 μm,but preferably is about 3.9 μm. Between the center of the fiber and theradial position at 3.9 μm, inner core 12 includes arefractive-index-increasing dopant such as GeO₂ or the like thatproduces a peak index of refraction at or near the axial center of fiber10 and a minimum for the inner core at its outer radius. At the peak,the refractive index difference for inner core 12 is about 0.0082 to0.0095, but preferably is about 0.0085. The concentration of therefractive-index-increasing dopant decreases from the center of innercore 12 to the outer radius at about 3.9 μm in a manner to produce aprofile having a curved slope that resembles a substantially parabolicshape. The preferred substantially parabolic shape corresponds to aprofile α of between about 1.7 and 2.0, but preferably of about 1.9. Ingeneral, the profile of inner core 12 is a profile α corresponding tothe following: $\begin{matrix}{{{\Delta \quad n} = {\Delta \quad {n_{1}\lbrack {1 - ( \frac{r}{r1} )^{\alpha}} \rbrack}}},{r \in \lbrack {0,{r1}} \rbrack}} & (5)\end{matrix}$

As is readily known to one of ordinary skill in the art, the profile αindicates the amount of roundness or curvature to the profile of thecore, where α=1 corresponds to a triangular shape for the glass core,and α=2 corresponds to a parabola. As the value of α becomes greaterthan 2 and approaches 6, the refractive index profile becomes morenearly a step index profile. A true step index is described by an α ofinfinity, but an α of about 4 to 6 is a step index profile for practicalpurposes. The profile α may have an index depression, in the shape of aninverted cone, along its centerline, for example if the fiber isproduced by the OVD or MCVD methods.

First glass layer 14 has a refractive index difference Δn2, referencedas 24, that is less than Δn1. As shown in FIG. 2, the preferredrefractive index difference Δn2 for first glass layer 14 has a constantvalue of about 0, which corresponds to a layer of pure SiO₂ glass.However, the refractive index difference Δn2 of first glass layer maydiffer from zero, due to the presence of refractive index modifyingdopants, provided that the dopant content of first glass layer 14 islow. It is envisaged that the refractive index difference varies acrossfirst glass layer. In any case, refractive-index-modifying dopants frominner core 12 or from second glass layer 16 may diffuse into first glasslayer 14 during fiber fabrication.

Applicants have determined that, in order to achieve the above describedadvantages in combination with a relatively high field intensity infirst glass layer 14, e.g., in terms of fiber low loss and lownon-linearity, a low-dopant content in first glass layer 14 correspondsto a dopant content such as to cause a refractive index difference Δn2for first glass layer 14 (in absolute value) of about, or preferablylower than, 15% of the fiber peak refractive index difference, i.e. ofthe refractive index difference Δn3 of second glass layer 16. Theskilled in the art can adapt this value so that the resulting opticalfiber has non-linear and/or loss characteristics matching thecharacteristics of an optical system that he/she intends to make, suchas length of the optical transmission line, amplifier number and spacingand/or power, number and wavelength spacing of the transmission signals.

According to a preferred embodiment, improved fiber characteristics canbe achieved by a dopant concentration in first glass layer 14 such as tocause a refractive index difference Δn2 that is lower in absolute valuethan 10% of refractive index difference Δn3 of second glass layer 16.This low dopant content in first glass layer, in combination with arelatively high field intensity in that region, gives a very limitedcontribution to the fiber non-linearity coefficient and loss.

Still more preferred fiber characteristics can be achieved by arefractive index difference Δn2 lower, in absolute value, than 5% ofrefractive index difference Δn3 of second glass layer 16.

First glass layer 14 has an outer radius r2 which, as shown in FIG. 2,is between about 9.0 μm and 12.0 μm, but preferably is 9.2 μm. As aresult, first glass layer 14 has a width extending from about 4.8 μm toabout 8.4 μm for a first embodiment of the present invention.

Second glass layer 16, like inner core 12, has its refractive indexdifference increased by doping the width of the glass layer with GeO₂and/or other well-known dopants. Second glass layer 16 has asubstantially parabolic profile across its radius that culminates in amaximum refractive index difference Δn3, depicted as 22 in FIG. 2, thatexceeds the maximum refractive index difference Δn1 of glass core 12 andrefractive index difference Δn2 of first layer 14. Index profiles otherthan parabolic, e.g., rounded or step like, are also envisaged forsecond glass layer 16.

Preferably the index of refraction Δn3 of second glass layer 16 at itspeak exceeds the peak index of refraction Δn1 for inner core 12 by morethan 5%. The index of refraction Δn3 of second glass layer 16 at itspeak is about 0.009 to 0.012, but preferably is about 0.0115. Secondglass layer 16 has a width w that equals about 0.6 μm to 1.0 μm, butpreferably is about 0.9 μm.

Cladding 18 of optical fiber 10 has a refractive index profile 26 thathas a refractive index difference substantially equal to 0. Asmentioned, cladding 26 preferably is pure SiO₂ glass but may includedopants that do not raise its index of refraction above that of themaximum indices of refraction 20 and 22 of inner core 12 and secondlayer 16.

Applicants have found that optical transmission fiber 10 with therefractive index profile of FIG. 2 has several desirable opticalcharacteristics for use in WDM transmission. Preferably, opticaltransmission fiber 10 is used in a transmission system that operatesover a wavelength range of 1530 nm to 1565 nm where the fiber provides atotal dispersion of 5 to 10 ps/nm/km across that operating wavelengthrange. More particularly, fiber 10 exhibits in the above wavelengthrange the following optical characteristics, with the characteristics ofthe most preferred embodiment in parentheses:

Dispersion=5-10 ps/nm/km (5.65 ps/nm/km @1550 nm)

Dispersion Slope @1550 nm≦0.06 ps/nm²/km (0.056 ps/nm²/km)

Macrobending Attenuation Coefficient @1550 nm<1 dB/km

Effective Area>45 μm²

γ<2 W⁻¹ km⁻¹ (1.4 W⁻¹ km⁻¹ @1550 nm)

λ_(cutoff)<1480 nm (fiber cutoff wavelength according to ITU.T G.650)

These optical characteristics satisfy desired qualities for atransmission fiber for WDM systems both of soliton and non-soliton type.

As mentioned, the non-linearity coefficient γ provides an indication ofthe susceptibility of a fiber to non-linear effects. With γ of less than2 W⁻¹ km⁻¹, fiber 10 exhibits favorable response in high power opticaltransmission systems that may otherwise initiate severe problems fromself-phase modulation, cross-phase modulation, and the like. As well,fiber 10 includes a nonzero dispersion value across the operating rangeof 1530 nm to 1565 nm, which helps to deter detrimental four-wavemixing. Moreover, the relatively small slope of total dispersion acrossthe operating wavelength range enables fiber 10 to provide relativelysmall differences of dispersion between carrier wavelengths in a WDMsystem.

FIGS. 3-6 more fully illustrate the relationships between the physicaland optical characteristics of fiber 10. These figures present resultsfrom computer simulations for fiber 10 for various physical and opticalrelationships when considering six parameters: radius r1 of inner core12, maximum index of refraction Δn1 of inner core 12, profile shape αfor inner core 12, outer radius r2 of first layer 14, width w of secondlayer 16, and maximum index of refraction Δn3 of second layer 16. In thesimulations represented by each of the graphs of FIGS. 3-6, these sixparameters were varied essentially at random substantially across theranges for the six parameters outlined above, i.e. r1 of 3.6-4.2 μm, Δn1of 0.0082-0.0095, α of 1.7-2.0, r2 of 9.0-12.0 μm, w of 0.6-1.0 μm, andΔn3 of 0.009-0.012. Each dot represents a different set of the sixparameters. The simulation considered only parameter sets havingΔn1<Δn3. Accordingly, all dots correspond to fibers having an outerrefractive index peak higher than the inner peak.

As shown in the simulation results of FIGS. 3-6, to achieve an opticalfiber having a low non-linearity factor, the area of the refractiveindex profile for inner core 12 should be lowered. An outer ring ofincreased refractive index, specifically second glass layer 16, is addedto help obtain a high effective area and a low non-linearity coefficientfor fiber 10. In particular, Applicants have found that the addition ofthe second glass layer of increased index of refraction heightens theelectrical field distribution in the cross-section of the fiber inregions with low dopant content, lowering it at the center of the fiber,so that the non-linearity coefficient γ remains low.

Furthermore, Applicants have found that the addition of the second glasslayer of increased index of refraction has low influence on overallfiber dispersion, and that fiber dispersion is essentially determined bythe radial dimension r1 of refractive index profile of inner core 12.

FIG. 3 illustrates the relationship between the radius r1 and thedispersion for fiber 10. The value of r1 is preferably less than 3X, toachieve a monomodal behavior at a given wavelength λ. For a given rangeof dispersion, a proper range of radial dimension r1 for the refractiveindex profile may be determined.

For deterring non-linear effects and enabling larger power the effectivearea of fiber 10 should be kept relatively high, preferably in excess of45 μm². It is possible to lower the non-linearity coefficient in twoways: either reducing the area of the refractive index profile for theinner core (i.e., the area of the region between peak 20 and theco-ordinate axes in FIG. 2) (FIGS. 4-5), or increasing the refractiveindex of the second outer peak (FIG. 6). FIGS. 4 and 5 show the formereffect for a series of computer simulations. For the sake of clarity,the radial dimension r1 in the simulations was kept constant, and sodispersion is essentially determined, in these figures. In order toreduce the area of the refractive index profile for the inner core, itis useful to reduce the refractive index difference Δn1 for a givenradial dimension r1. An increase in effective area as the refractiveindex Δn1 is lowered occurs as shown in FIG. 4 because electric fieldconfinement in inner core 12 becomes weaker.

Because a decrease in the area of the refractive index profile for theinner core leads to an increased effective area for the fiber, thedecrease in area also provides a lower non-linearity coefficient γ, asshown in FIG. 5. Thus, the fiber 10 with a lower non-linearitycoefficient γ can handle increased power and/or have decreasednon-linear effects.

As well, Applicants have recognized that the addition of a lateral areaof higher index of refraction positioned radially outward from the innercore will help to obtain a relatively large effective area and thereforelow non-linearity coefficient γ. The addition of this lateral peakrefractive index zone helps to make the electrical field distributionlarger but does not substantially affect dispersion.

The radial position of second layer 16, its width, and its peak index ofrefraction all affect the overall effective area of the fiber. Forexample, FIG. 6 shows results of a computer simulation comparingeffective area and the peak index of refraction difference for secondlayer 16, where other fiber parameters are kept constant for the sake ofclarity. As is evident from FIG. 6, an increasing index of refractiondifference for the outer ring 16 generates an increasing effective areafor fiber 10.

FIG. 7 illustrates the spread of electric field within the cross-sectionof fiber 10 due to the addition of outer ring 16. In FIG. 7, references20 and 22 denote an inner core and an outer ring, respectively, whilereference 23 denotes the electrical field distribution across the fiberradius. The presence of the outer peak enlarges the electric fielddistribution in the fiber.

Applicants have also determined that optical fibers having a maximumrefractive index region in an outer ring of the core as in the profileof FIG. 2 exhibit a low A_(eff)·γ product, i.e., a lower γ in comparisonwith other fibers having the same effective area. For example, FIG. 8Aillustrates the simulated relationship between γ and effective area forfibers 10 according to the first embodiment. In contrast, FIG. 8Billustrates the simulated relationship between γ and effective area forconventional dual-shape dispersion-shifted optical fibers, which exhibita less desirable (i.e., a higher) A_(eff)·γ product.

In short, fiber 10 provides an optical waveguide with a uniquerefractive index profile for transmitting optical WDM signal withnonzero dispersion and a relatively low non linearity coefficient. Thesefeatures enable fiber 10 to minimize signal degradation due to four-wavemixing and/or to permit the use of higher power.

FIG. 9 illustrates a second embodiment of the present invention foroptical fiber 10 of FIG. 1. In this second embodiment, inner core 12 hasa radius r1 that is about 2.3 μm to 3.6 μm, but preferably is about 2.77μm. Between the center of the fiber and the radial position at 2.77 μm,inner core 12 includes one or more refractive-index-increasing dopants,such as GeO₂ or the like, that produce a peak index of refraction at ornear the axial center of fiber 10 and a minimum for the inner core atits outer radius. At the peak, the index of refraction Δn1 for innercore 12 in the second embodiment is about 0.010 to about 0.012, andpreferably is about 0.0113. As with the first embodiment, theconcentration of the refractive-index-modifying dopant in the core 12decreases from the center to the outer radius at about 2.77 μm in amanner to produce a profile having a profile α of about 1.4 to about3.0, but preferably of about 2.42. First glass layer 14 in the secondembodiment has a substantially constant refractive index difference Δn2,referenced as 24, that is about 0, due to undoped silica glass. However,as previously explained with reference to the first embodiment of FIG.2, low dopant concentrations can be present in first glass layer 14. Thefirst layer 14 extends to an outer radius r2 equal to between about 4.4μm and 6.1 μm, but preferably equal to 5.26 μm. As a result, first glasslayer 14 has a width extending from about 0.8 μm to about 3.8 μm, butpreferably of about 2.49 μm, for a second embodiment of the presentinvention.

As with the first embodiment, the second embodiment includes a secondglass layer 16, like inner core 12, with its refractive index differenceincreased by doping the width of the glass layer with GeO₂ and/or otherwell-known refractive-index-increasing dopants. Second glass layer 16has a substantially parabolic profile across its radius that culminatesin a maximum refractive index difference Δn3, depicted as 22 in FIG. 9.Index profiles other than parabolic, e.g., rounded or step like, arealso envisaged for second glass layer 16.

Preferably the index of refraction Δn3 of second glass layer 16 at itspeak exceeds the peak index of refraction Δn1 for inner core 12 by morethan 5%. The index of refraction Δn3 of second glass layer 16 at itspeak is about 0.012 to 0.014, but preferably is about 0.0122.

Second glass layer 16 has a width w that equals about 1.00 μm to about1.26 μm, but preferably is about 1.24 μm.

Preferably, optical transmission fiber 10 is used in a transmissionsystem that operates over a wavelength range of 1530 nm to 1565 nm wherethe fiber provides positive nonzero dispersion characteristics. Nonzerodispersion fibers are described in ITU-T Recommendation G.655.

Fiber 10 constructed according to the second embodiment of FIG. 9exhibits the following preferred optical characteristics (the values aregiven for a value of 1550 nm, unless otherwise indicated):

Chromatic Dispersion @1530 nm≧0.5 ps/nm/km

0.07 ps/nm²/km≦Dispersion Slope≦0.11 ps/nm²/km

45 μm²≦A_(eff)≦100 μm²

1 W⁻¹ km⁻¹≦γ≦2 W⁻¹ km⁻¹

Macrobending Attenuation Coefficient≦0.01 dB/km (fiber loosely wound in100 turns with a bend radius of 30 mm)

Microbending Sensitivity≦10 (dB/km)/(g/mm)

λ_(cutoff)≦1600 nm (fiber cutoff wavelength according to ITU.T G.650)

The second embodiment for fiber 10 having the above-listed opticalcharacteristics provides acceptable conditions for the transmission ofboth solitons and non-soliton WDM systems.

FIG. 10 depicts a refractive index profile of a third embodiment of thepresent invention for optical fiber 10, whose cross-section is shown inFIG. 1. The third embodiment, like the first and second embodiments,includes an inner core with a heightened refractive index difference Δn1and a profile shape α, together with a first layer of glass having alower refractive index difference Δn2 and a second layer of glass havingthe maximum index of refraction difference Δn3 in the cross-section ofthe fiber. The following sets forth the preferred physical parametersfor fiber 10 according to the third embodiment of the present inventionas illustrated in FIG. 10.

Inner Core Radius r1=2.387 μm

Inner Core Refractive Index Difference Δn1=0.0120

First Layer Radius r2=5.355 μm

First Layer Refractive Index Difference Δn2=0.0

Second Layer Width w=1.129 μm

Second Layer Refractive Index Difference Δn3=0.0129.

Of course, variations from these optimal structural values do not altertheir general inventive features. Fiber 10 according to the thirdembodiment of the present invention advantageously obtains the followingoptimal optical characteristics (at a wavelength of 1550 nm):

Dispersion=3.4 ps/nm/km

Dispersion Slope=0.11 ps/nm²/km

Mode Field Diameter=9.95 μm

Effective Area=90 μm²

γ=1.00 W⁻¹ km⁻¹.

The third embodiment for fiber 10 having the above-listed opticalcharacteristics provides acceptable conditions for the transmission inboth solitons and non-soliton WDM systems.

FIG. 11 illustrates a fourth refractive index profile for optical fiber10 that generates optical characteristics of nonzero positivedispersion. The physical characteristics of the inventive fiber of FIG.11 include a radius r1 for inner core 12 of about 3.2 μm, an index ofrefraction profile α for inner core 12 of about 2.9, a maximumrefractive index difference Δn1 at reference 20 for inner core 12 ofabout 0.0088, an outer radius of first glass layer 14 of about 7.2 μmwith an index of refraction Δn2 at reference 24 of about 0, a width ofsecond glass layer 16 of about 0.8 μm, and a maximum index of refractionΔn3 at reference 22 of the second glass layer 16 of about 0.0119. Aswith the refractive index profile of FIG. 2, the profile of FIG. 11 forthe nonzero positive dispersion fiber has the characteristic multiplepeaks high refractive index, where the outer peak is present in thesecond glass layer 16, has a substantially parabolic shape, and at itsmaximum 22 exceeds the maximum index of refraction 20 within inner core12.

Fiber 10 having the refractive index profile of FIG. 11 providespositive total fiber dispersion across the operating wavelength band of1530 nm to 1565 nm. Such a performance has desirable application inoptical systems that have a relatively high optical power and wouldotherwise generate deleterious four-wave mixing products. FIG. 12depicts the simulated total dispersion vs. wavelength for optical fiber10 having the refractive index profile of FIG. 11. As shown in thisfigure, the refractive index profile of FIG. 11 produces dispersionacross the wavelength band of about 1530 nm to 1565 nm that stretchesbetween about 0.76 ps/km/nm and 3.28 ps/km/nm. Specifically, the fiberwith the refractive index profile shown in FIG. 11 provides thefollowing optical characteristics at 1550 nm:

Dispersion=2.18 ps/nm/km

Dispersion Slope=0.072 ps/nm²/km

Macrobending Attenuation Coefficient=0.01 dB/km

Mode Field Diameter=9.0 μm

Effective Area=62 μm²

γ=1.8 W⁻¹ km⁻¹.

All of these characteristics fall within the ranges stated by ITU-TG.655 Recommendation for nonzero dispersion fibers.

FIG. 13 illustrates a fifth refractive index profile for optical fiber10 that generates optical characteristics of nonzero negative dispersionwith relatively low non-linearity coefficient. The physicalcharacteristics of the inventive fiber of FIG. 13 include a radius r1for inner core 12 of about 2.4 μm to 3.2 μm and preferably of about 2.6μm, an index of refraction profile α for inner core 12 of about 1.8 to3.0 and preferably of about 2.48, a maximum refractive index differenceΔn1 at reference 20 for inner core 12 of about 0.0106-0.0120 andpreferably of about 0.0116, an outer radius of first glass layer 14 ofabout 5.3 μm to 6.3 μm and preferably of about 5.9 μm with an index ofrefraction Δn2 at reference 24 preferably of about 0, a width of secondglass layer 16 of about 1.00 μm to 1.08 μm and preferably of about 1.08μm, and a maximum index of refraction Δn3 at reference 22 of the secondglass layer 16 of about 0.0120 to 0.0132 and preferably of about 0.0129.As previously explained, low dopant concentrations can be present infirst glass layer 14. As with the refractive index profile of FIGS. 2,9, 10, and 11, the profile of FIG. 13 for the nonzero negativedispersion fiber has the characteristic multiple peaks of highrefractive index, where the outer peak is present in the second glasslayer 16, has a substantially parabolic shape, and at its maximum 22exceeds the maximum index of refraction 20 within inner core 12. Indexprofiles other than parabolic, e.g., rounded or step like, are alsoenvisaged for second glass layer 16. Preferably the index of refractionΔn3 of second glass layer 16 at its peak exceeds the peak index ofrefraction Δn1 for inner core 12 by more than 5%.

Fiber 10 having the refractive index profile of FIG. 13 providesnegative total fiber dispersion across the operating wavelength band of1530 nm to 1565 nm. Such a performance has desirable application inoptical systems used in underwater transmission systems that have arelatively high optical power and would otherwise generate deleteriousfour-wave mixing products. Specifically, the fiber with the refractiveindex profile shown in FIG. 13 provides the following opticalcharacteristics at 1550 nm, with the characteristics of the mostpreferred embodiment in parentheses:

Dispersion≦−0.5 ps/nm/km (−2.46 ps/nm/km)

0.07 ps/nm²/km≦Dispersion Slope≦0.12 ps/nm²/km (0.11 ps/nm²/km)

Macrobending Attenuation Coefficient≦0.01 dB/km (0.0004 dB/km)

Mode Field Diameter=9.1 μm

45 μm²≦Effective Area≦75 μm² (68 μm²)

1.2 W⁻¹ km⁻¹≦γ≦2 W⁻¹ km⁻¹ (1.3 W⁻¹ km⁻¹)

λ_(cutoff)≦1600 nm (fiber cutoff wavelength according to ITU.T G.650)

A sixth refractive index profile for optical fiber 10 will now bedescribed that generates optical characteristics of shifted dispersionwith relatively low non-linearity coefficient. Dispersion shifted fibersare described in ITU-T Recommendation G.653. The physicalcharacteristics of the fiber according to the sixth embodiment include aradius r1 for inner core 12 of about 3.2 μm, an index of refractionprofile α for inner core 12 of about 2.8, a maximum refractive indexdifference Δn1 at reference 20 for inner core 12 of about 0.0092, anouter radius of first glass layer 14 of about 7.8 μm with an index ofrefraction Δn2 of about 0, a width of second glass layer 16 of about 0.8μm, and a maximum index of refraction Δn3 of the second glass layer 16of about 0.0118. As with the refractive index profile of FIGS. 2, 9, 10,11 and 13 the profile according to the sixth embodiment for thedispersion shifted fiber has the characteristic multiple peaks of highrefractive index, where the outer peak is present in the second glasslayer 16, has a substantially parabolic shape, and at its maximum 22exceeds the maximum index of refraction 20 within inner core 12. Indexprofiles other than parabolic, e.g., rounded or step like, are alsoenvisaged for second glass layer 16. Preferably the index of refractionΔn3 of second glass layer 16 at its peak exceeds the peak index ofrefraction Δn1 for inner core 12 by more than 5%.

Fiber 10 having the refractive index profile of FIG. 13 provides lowabsolute value total fiber dispersion across the operating wavelengthband of 1530 nm to 1565 nm.

Specifically, the fiber provides the following optical characteristics,given at 1550 nm unless otherwise indicated:

Dispersion=0.42 ps/nm/km

Dispersion Slope=0.066 ps/nm²/km

Dispersion @1525 nm=−1.07 ps/nm/km

Dispersion @1575 nm=+2.22 ps/nm/km

Macrobending Attenuation Coefficient=0.6 dB/km

Mode Field Diameter=8.8 μm

Effective Area=58 μm²

γ=1.56 W⁻¹ km⁻¹

λ_(cutoff)=1359 nm (fiber cutoff wavelength according to ITU.T G.650)

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the system and method of thepresent invention without departing from the spirit or scope of theinvention. For example, the refractive index profiles depicted in thefigures are intended to be exemplary of preferred embodiments. Theprecise shape, radial distance, and refractive index differences mayreadily be fluctuated by one of ordinary skill in the art to obtain theequivalent fibers as disclosed herein without departing from the spiritor scope of this invention. Although fiber operation in a wavelengthrange of between 1530 nm and 1565 nm has been disclosed for the givenembodiments, signals in different wavelength ranges can be transmittedin a fiber according to the invention, if specific wavelengthrequirements arise in present or future optical communication systems.In particular the skilled in the art may envisage use of the describedfibers, or of straightforward modifications thereof, to operate in anextended wavelength range of between about 1520 nm and about 1620 nm,where silica keeps low attenuation properties.

The present invention covers the modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents.

What is claimed is:
 1. An optical transmission fiber with a lownon-linearity coefficient γ and high effective area for use in anoptical transmission system, comprising: a core region comprising: aglass inner core having a first maximum refractive index difference Δn1,a profile α, and a radius r1; a first glass layer radially surroundingthe inner core, having a refractive index difference Δn2 less than Δn1,and having an outer radius r2; and a second glass layer radiallysurrounding the first layer, having a second maximum refractive indexdifference Δn3 greater than Δn1, and having a width w, and a low losscladding surrounding said core region, wherein said non-linearitycoefficient γ is less than about 2 W⁻¹ km⁻¹, characterized in that saidrefractive index difference Δn2 is lower in absolute value than 10% ofsaid second maximum refractive index difference Δn3.
 2. The opticaltransmission fiber of claim 1, wherein Δn2 is lower in absolute valuethan 5% of Δn3.
 3. The optical transmission fiber of claim 2, whereinΔn2 is about 0.0.
 4. The optical transmission fiber of claim 1, whereinthe maximum refractive index difference Δn3 of second glass layerexceeds the maximum core refractive index difference Δn1 by more than5%.
 5. The optical transmission fiber of claim 1, wherein r1 is about3.6 μm to 4.2 μm, r2 is about 9.0 μm to 12.0 μm, and w is about 0.6 μmto 1.0 μm.
 6. The optical transmission fiber of claim 5, wherein α isabout 1.7 to 2.0.
 7. The optical transmission fiber of claim 5 or 6,wherein Δn3 is about 0.009 to 0.012.
 8. The optical transmission fiberof claim 7, wherein Δn1 is about 0.0082 to 0.0095.
 9. The opticaltransmission fiber of any one of claims 1, 5 or 6 wherein totaldispersion for the fiber in a wavelength range of 1530 nm to 1565 nm isabout 5 ps/nm/km to 10 ps/nm/km.
 10. The optical transmission fiber ofclaim 1, wherein r1 is about 2.3 μm to 3.6 μm, r2 is about 4.4 μm to 6.1μm, and w is about 1.00 μm to 1.26 μm.
 11. The optical transmissionfiber of claim 10, wherein α is about 1.4 to 3.0.
 12. The opticaltransmission fiber of any one of claims 1, 10 or 11 wherein totaldispersion for the fiber in a wavelength range of 1530 nm to 1565 nm isgreater than about 0.5 ps/nm/km.
 13. The optical transmission fiber ofclaim 10 or 8, wherein Δn3 is about 0.0120 to 0.0140.
 14. The opticaltransmission fiber of claim 13, wherein Δn1 is about 0.0100 to 0.0120.15. The optical transmission fiber of claim 13, wherein Δn2 is lower inabsolute value than 5% of Δn3.
 16. The optical transmission fiber ofclaim 1, wherein r1 is about 2.4 μm to 3.2 μm, r2 is about 5.3 μm to 6.3μm, and w is about 1.00 μm to 1.08 μm.
 17. The optical transmissionfiber of claim 16, wherein α is about 1.8 to 3.0.
 18. The opticaltransmission fiber of any one of claims 1, 16, or 17, wherein totaldispersion for the fiber in a wavelength range of 1530 nm to 1565 nm isless than about −0.5 ps/nm/km.
 19. The optical transmission fiber ofclaim 16 or 17, wherein Δn3 is about 0.0120 to 0.0132.
 20. The opticaltransmission fiber of claim 19, wherein Δn1 is about 0.0106 to 0.0120,Δn2 is about 0.0.
 21. The optical transmission fiber of claim 19,wherein Δn2 is lower in absolute value than 5% of Δn3.
 22. An opticaltransmission system comprising an optical transmitter for outputting anoptical signal and an optical transmission line for transmitting saidsignal, characterized in that the optical transmission line comprises anoptical transmission fiber comprising a core region and a low losscladding surrounding the core region, the core region having a firstrefractive index peak in the central cross-sectional area of the coreregion, an outside ring having a second refractive index peak higherthan the first peak and a low-dopant-content region between the twopeaks having a third refractive index lower in absolute value than 15%of the second refractive index peak.
 23. An optical transmission systemaccording to claim 22, wherein said low-dopant-content region has arefractive index difference, in absolute value, equal to or lower than15% of the fiber peak refractive index difference.
 24. An opticaltransmission system according to claim 22, further comprising: aplurality of optical transmitters for outputting a plurality of opticalsignals, each signal having a particular wavelength; an optical combinerfor combining the optical signals to form a wavelength divisionmultiplexed optical communication signal and outputting the combinedsignal onto said optical transmission line.
 25. An optical transmissionsystem according to claim 22, wherein said optical transmission fiberhas a length greater than 50 km.
 26. An optical transmission systemaccording to claim 22, wherein said optical transmission line comprisesan optical amplifier.
 27. An optical transmission fiber with a higheffective area and a non-linearity coefficient γ lower than about 2 W⁻¹km⁻¹ for use in an optical transmission system, comprising: a coreregion comprising: a glass inner core having a first maximum refractiveindex difference Δn1, a profile α, and a radius r1; a first glass layerradially surrounding the inner core, having a refractive indexdifference Δn2 less than Δn1, and having an outer radius r2; and asecond glass layer radially surrounding the first layer, having a secondmaximum refractive index difference Δn3 greater than Δn1, and having awidth w, and a low loss cladding surrounding said core region,characterized in that said first glass layer comprises alow-dopant-content region.
 28. A method for controlling non-lineareffects in optical fiber transmission comprising the steps of:generating an optical signal; coupling the optical signal in a silicaoptical fiber having a non-linearity coefficient and a centralcross-sectional area with a first refractive index peak; enhancing afield intensity associated with the optical signal in a fibercross-sectional area outside said central cross-sectional area, thefiber having a doped annular glass ring with a second refractive indexpeak value, higher than the first peak, the annular glass ring beingsurrounded by a low loss cladding; and a fiber cross-sectional regionbetween the two peaks with a refractive index lower in absolute valuethan 15% of the second refractive index peak so as to reduce the fibernon-linearity coefficient.